In recent years, optical brain function imaging devices (light measuring devices) for a simple noninvasive measurement using light have been developed in order to observe brain activity. In such an optical brain function imaging device, the brain is irradiated with near infrared rays having three different wavelengths λ1, λ2 and λ3 (780 nm, 805 nm and 830 nm, for example) from the light-transmitting probes placed on the surface of the head of the subject, and at the same time, the intensity A(λ1), A(λ2) and A(λ3) (information on the amount of received light) of the near infrared rays of each wavelength λ1, λ2 and λ3 diffused from the brain is respectively detected by the light-receiving probes placed on the surface of the head.
In order to find the product [oxyHb] of the concentration of oxyhemoglobin in the cerebral blood vessels and the length of the light path and the product [deoxyHb] of the concentration of deoxyhemoglobin in the cerebral blood vessels and the length of the light path from the thus-gained information on the amount of received light A (λ1), A (λ2) and A (λ3), the Modified Beer-Lambert Law has been used to prepare simultaneous equations shown in the relational expressions (1), (2) and (3), for example, and these simultaneous equations have been solved (see Non-Patent Document 1). Furthermore, the product ([oxyHb]+[deoxyHb]) of the concentration of total hemoglobin and the length of the light path has been calculated from the product [oxyHb] of the concentration of oxyhemoglobin and the length of the light path and the product [deoxyHb] of the concentration of deoxyhemoglobin and the length of the light path.A(λ1)=EO(λ1)×[oxyHb]+Ed(λ1)×[deoxyHb]  (1)A(λ2)=EO(λ2)×[oxyHb]+Ed(λ2)×[deoxyHb]  (2)A(λ3)=EO(λ3)×[oxyHb]+Ed(λ3)×[deoxyHb]  (3)
Here, EO(λm) is the coefficient of absorbance of the oxyhemoglobin for the light of the wavelength λm, and Ed(λm) is the coefficient of absorbance of the deoxyhemoglobin for the light of the wavelength λm.
Here, the relationship between the distance between a light-transmitting probe and a light-receiving probe (channel) and the portion to be measured is described. FIG. 5(a) is a cross-sectional diagram showing the relationship between a pair of probes, a light-transmitting probe and a light-receiving probe, and the portion to be measured, and FIG. 5(b) is a plan diagram of FIG. 5(a).
A light-transmitting probe 12 is pressed against a light transmitting point T on the surface of the head of a subject, and at the same time, a light-receiving probe 13 is pressed against a light receiving point R on the surface of the head of the subject. Thus, light is emitted from the light-transmitting probe 12, and at the same time, the light diffused and absorbed from the surface of the head enters into the light-receiving probe 13. At this time, the light that has passed through the banana-shaped area (area to be measured) from among the light emitted from the light transmitting point T on the surface of the head reaches the light receiving point R on the surface of the head. As a result, information on the amount of received light A (λ1), A (λ2) and A (λ3) concerning the portion to be measured S of the subject at a depth of approximately L/2, which is half of the distance along the line connecting the light transmitting point T and the light receiving point R along the surface of the head of the subject from the mid-point M of the line L connecting the light transmitting point T and the light receiving point R along the surface of the head of the subject, is particularly gained from among the area to be measured.
In optical brain function imaging devices, a near infrared spectrometer, for example, is used in order to measure the change in the product [oxyHb] of the concentration of oxyhemoglobin and the length of the light path, the change in the product [deoxyHb] of the concentration of deoxyhemoglobin and the length of the light path, and the change in the product ([oxyHb]+[deoxyHb]) of the concentration of total hemoglobin and the length of the light path concerning a number of portions to be measured in the brain (see Patent Document 1).
FIG. 6 is a block diagram schematically showing an example of the structure of a conventional near infrared spectrometer. Here, several optical fibers for transmitting light and several optical fibers for receiving light are omitted in order to simplify the drawing.
A near infrared spectrometer 201 has a housing 11 in a rectangular parallelepiped form.
A light source 2 for emitting light, a light source driving mechanism 4 for driving the light source 2, a light detector 3 for detecting light, an A/D converter 5, a control unit 21 for transmitting and receiving light, a control unit 22 for analysis and a memory 23 are provided inside the housing 11, and at the same time, 64 light-transmitting probes 12, 64 light-receiving probes 13, 64 optical fibers 14 for transmitting light, 64 optical fibers 15 for receiving light, a display 26 having a monitor screen 26a, and a keyboard (input device) 27 are provided outside the housing 11.
The light source driving mechanism 4 drives the light source 2 using a drive signal inputted from the control unit 21 for transmitting and receiving light. The light source 2 is made of semiconductor lasers LD1, LD2, LD3 and the like that can emit near infrared rays having three different wavelengths λ1, λ2 and λ3, for example.
The light detector 3 is a detector for outputting a light reception signal (information on the amount of received light) A (λ1), A (λ2) or A (λ3) to the control unit 21 for transmitting and receiving light via the A/D converter 5 by detecting the respective near infrared rays and is a photomultiplier, for example.
The optical fibers 14 for transmitting light and the optical fibers 15 for receiving light are flexible lines with a diameter of 2 mm and a length of 2 meters to 10 meters and can convey near infrared rays in the direction of the axis so that the near infrared rays that have entered through one end pass through the inside so as to emit through the other end.
One optical fiber 14 for transmitting light is connected to one probe 12 for transmitting light and one semiconductor laser LD1, LD2 or LD3 in the light source 2 at the two ends so that the two are away from each other at a set length (2 meters to 10 meters).
One optical fiber 15 for receiving light is connected to one probe 13 for receiving light and one photomultiplier in the light detector 3 at the two ends so that the two are away from each other at a set length (2 meters to 10 meters).
In this near infrared spectrometer 201, a holder 30 is used in order to make the 64 light-transmitting probes 12 and the 64 light-receiving probes 13 make contact with the surface of the head of a subject in a predetermined alignment. FIG. 7 is a plan diagram showing an example of the holder 30 into which 64 light-transmitting probes and 64 light-receiving probes are inserted.
The light-transmitting probes 12T1 to 12T64 and the light-receiving probes 13R1 to 13R64 are aligned alternately in a matrix of 16 in the longitudinal direction and 16 in the lateral direction. As a result, the distance between the light-transmitting probes 12 and the light-receiving probes 13 is constant so that the information on the amount of received light A (λ1), A (λ2) and A (λ3) is gained at a certain depth from the surface of the head. Here, a channel of 30 mm is generally used, and in the case where the channel is 30 mm, it is possible for the information on the amount of received light A (λ1), A (λ2) and A (λ3) to be gained at a depth of 15 mm to 20 mm from the mid-point of each channel. That is to say, the locations at a depth of 15 mm to 20 mm from the surface of the head approximately correspond to the portions on the surface of the brain, and thus, the amount of received light A (λ1), A (λ2) and A (λ3) concerning the brain activity is gained.
The curvature of the surface of the head differs depending on the sex, the age and the individual, and therefore, a holder that can be easily worn on the head with a different curvature of the surface has been proposed. In the holder, support portions for holding the light-transmitting probes 12 and the light-receiving probes 13 are aligned in a tetragonal lattice on the surface of the head, and the support portions are linked at a set distance (30 mm, for example) with connection portions that do not exhibit stretchability, and furthermore, the connection portions are rotatable by a predetermined angle or less with the support portions as a rotational axis in a plane through which the holder makes contact with the surface of the head (see Patent Document 2).
This holder 30 is provided with 128 socket parts 33 for fixing the light-transmitting probes 12 and the light-receiving probes 13, 232 connection parts 31 and 128 nut parts 32.
Here, FIG. 8 is an exploded perspective diagram showing a light-transmitting probe 12, a nut part 32, two connection parts 31 and a socket part 33, and FIG. 9 is a diagram showing the light-transmitting probe 12, the nut part 32, the two connection parts 31 and the socket part 33 after assembly.
The connection parts 31 are plates in an I-shape. In addition, the connection parts 31 have insertion portions 31a in annular form at the two ends and a linking portion 31b for linking the insertion portions 31a at the two ends at a set distance. Circular through holes through which a socket part 33 is inserted are created at the center of the respective insertion portions 31a. In addition, the linking portion 31b has a width of 10 mm and a thickness of 0.1 mm with a set distance of 31.5 mm, which is the distance between the centers of the through holes at the two ends. The linking portion 31 is formed so as to have flexibility only in the direction of the thickness. That is to say, the insertion portions 31a at the two ends are always held at a channel length X.
The socket part 33 has a main body portion 33a in cylindrical form, a flange 33b in annular form and a bottom 33c in annular form so that a light-transmitting probe 12 or a light-receiving probe 13 can be inserted inside, and at the same time, the outside of the main body portion 33a is threaded so that the nut part 32 can be engaged.
The nut part 32 is in annular shape having a circular through hole, and the inside is threaded so that the main body portion 33a of the socket part 33 can be engaged. Here, the size of the through hole is greater than that of the main body portion 33a of the socket part 33 and is smaller than that of the flange 33b of the socket part 33 as viewed from the top.
As a result, the insertion portion 31a of a connection part 31 can be sandwiched between the flange 33b of the socket part 33 and the nut part 32 so as to be fixed when the main body portion 33a of the socket part 33 is screwed into the nut part 32.
The light-transmitting probes 12 are in cylindrical form (diameter: 5 mm, for example), which can fix a socket part 33. An optical fiber 14 for transmitting light (diameter: 2 mm, for example) that is connected to the light source 2 is fixed to the inside of a light-transmitting probe 12 with a spring in between so that light can be emitted from the end of the optical fiber 14 for transmitting light.
In addition, the light-receiving probes 13 have the same structure as the light-transmitting probes 12 and are in cylindrical form (diameter: 5 mm, for example), which can fix a socket part 33. Thus, an optical fiber 15 for receiving light (diameter: 2 mm, for example) that is connected to the light detector 3 is fixed to the inside of a light-receiving probe 13 with a spring in between so that light can be received by the end of the optical fiber 15 for receiving light.
A holder 30, as in FIG. 7, is made of 128 socket parts 33, 232 connection parts 31 and 128 nut parts 32, for example. A doctor or another person who carries out the measurement slightly loosens the screw mechanism between the flange 33b of a socket part 33 and a nut part 32 so that one connection part 31 and another connection part 31 are fixed to each other by forming a desired angle around the socket part 33 as viewed from above, as shown in FIG. 9(a), and at the same time, the linking portion 31b of the connection part 31 can deform due to its flexibility, as shown in FIG. 9(b), so as to have a curvature that matches the surface of the head, and thus, this holder 30 can make close contact with the surface of the head when being worn. The doctor or the other person firmly fixes the screw mechanism between the flange 33b of the socket part 33 and the nut part 32 in the thus-deformed state. Then, the holder 30 does not return to a flat state, and as a result, the curvature is maintained. Finally, the doctor or the other person inserts the light-transmitting probes 12 and the light-receiving probes 13 into the socket parts 33 in a predetermined alignment.
In the positional relationship between these 64 light-transmitting probes 12T1 to 12T64 and 64 light-receiving probes 13R1 to 13R64, it is necessary to adjust the timing in the emissions of light from the light-transmitting probes 12 and the receptions of light by the light-receiving probes 13 so that one light-receiving probe 13 receives light emitted from only one light-transmitting probe 12 instead of receiving light emitted from a number of light-transmitting probes 12 simultaneously. Therefore, the memory 23 stores a control table showing the timing according to which the light source 2 emits light and the light detector 3 detects the light.
The control unit 21 for transmitting and receiving light where this control table is stored in the memory 23 outputs a drive signal for allowing one light-transmitting probe 12 to transmit light to the light source driving mechanism 4, and at the same time allows the light detector 3 to detect the light reception signal (information on the amount of received light) received by the light-receiving probe 13.
As a result, as shown in FIG. 7 as a plan view, a total of 232 pieces (S1 to S232) of information on the amount of received light A (λ1), A (λ2) and A (λ3) is collected.
Thus, the control unit 22 for analysis finds the change in the product [oxyHb] of the concentration of oxyhemoglobin and the length of the light path, the change in the product [deoxyHb] of the concentration of deoxyhemoglobin and the length of the light path, and the change in the product ([oxyHb]+[deoxyHb]) of the concentration of total hemoglobin and the length of the light path from the intensity of the light that has passed of each wavelength (wavelength of light absorbed by oxyhemoglobin and the wavelength of light absorbed by deoxyhemoglobin) by using the relational expressions (1), (2) and (3) on the basis of the total of the 232 pieces of information on the amount of received light A (λ1), A (λ2) and A (λ3).